This invention relates to liquid pumps, and more particularly to liquid pumps for delivering liquids at relatively high pressures under precise pressure regulation and control.
In the fluid handling field innumerable approaches have been devised for the delivery of pressurized liquids of widely ranging viscosities; one subset of this field encompasses linear drive or reciprocable pumps, usually including a piston and cylinder combination, with appropriate valving, wherein the piston is mechanically reciprocated within the cylinder to deliver liquids at predetermined pressures. Hydraulically driven motors have been used as the drive mechanism for the piston apparatus, wherein the output liquid pressure is a function of the hydraulic coil pressure delivered to the drive motor. Similarly, air-operated motors have been used as a power source, wherein the output liquid pressure is a function of the air pressure delivered to the air motor. In both of these instances, the drive motor operates until a balance exists between the input fluid pressure to the drive source and the output liquid pressure from the pump, whereupon a stall condition persists until one pressure or the other is varied. Systems of this type provide a convenient pressure control alternative by virtue of the inherent motor "stall" capability, but they suffer from the disadvantage of requiring externally powered sources of either air or hydraulic oil pressure. This requires additional equipment which is both expensive and bulky, and because of this pumps of this general class tend to be rather nonportable, being best adapted for fixed locations such as may be found in industrial plant locations.
Electrical drive motors have been used to power reciprocable pumps through intermediate crank shafts or gearing arrangements, particularly when it is desirable to construct such a pump which may be portable. The advantages of an electrical drive source lie in the fact that electrical power is readily available in practically all locations, and the pumping system may be designed in relatively compact form. Alternating current (AC) drive motors have been used in this application, together with pressure sensing devices coupled to the pump output pressure line, wherein the electric motor is turned on and off as the pump output liquid pressure varies. An example of this type of system is disclosed in U.S. Pat. No. 3,614,352, issued Oct. 14, 1971, and owned by the assignee of the present invention. In portable paint systems, pumps of this type have been coupled to paint delivery hoses of one hundred feet or more in length, with a paint spray gun connected to the end of the paint hose, and the pump has adequately delivered paint from a supply source through the spray gun for industrial and commercial painting applications. Typically, such paint pumps have been portable in nature, frequently being mounted on a wheeled frame assembly, and are in wide use by professional painting contractors and others engaged in the painting industry. Such systems have suffered from a disadvantage in the ability to control and regulate pressure, for the pressure control mechanism is typically designed to provide a fairly wide range between the pressure set points, activating the electric drive motor when paint pressure drops to a predetermined set point and shutting off the electric drive motor when the pressure rises to a second set point which may be 400-800 pounds per square inch (psi) higher than the low setpoint. This provides a delivery system wherein the output liquid, preferably paint, is delivered at widely fluctuating pressures, depending upon the "deadband" or on-off differential of the system. While this pressure fluctuation is adequate for many painting applications, it does affect the quality of atomization of paint being delivered by the system and in some applications adversely affects the quality of paint finish applied by the system. It is therefore desirable to reduce the pressure fluctuation range over which such systems operate. When the reduction in pressure on-off differential is attempted in prior art systems of the type described, the result is that the AC drive motor is turned on and off at frequent intervals. This frequently causes overheating of the motor, which can damage the motor and disable the system.
The problem of excessively high on-off differentials is also found in systems driven by air or hydraulic motors. In such systems, friction and other effects create excessively wide ranging pressure on-off differentials, which are difficult to correct in the design of the motor and the pressure regulation mechanism.
Direct current (DC) drive motors have also been used in reciprocable pumping systems to provide constant liquid pressure in a system driving multiple and variable liquid loads. An example of this type of system is shown in U.S. Pat. No. 3,985,467, issued Oct. 12, 1976, wherein two variable gain amplifier stages are used to drive a phase lock loop DC motor control circuit. The gain of one amplifier stage is controlled inversely to pump output pressure, and the gain of the second amplifier stage is controlled proportional to liquid flow through the pumping system. Such a system is used to drive a plurality of liquid outputs, one or more of which may be open at any given time. During system startup, when pressure and flow rate conditions are unstable, the two amplifier stages are circumvented by other special amplifier circuits, the aim of the invention being to maintain constant output pressure once stable output flow and pressure conditions exist.